The switched mode power supply (SMPS) is a well-known type of power converter having a diverse range of applications by virtue of its small size and weight and high efficiency, for example in personal computers and portable electronic devices such as cell phones. A SMPS achieves these advantages by switching one or more switching elements such as power MOSFETs at a high frequency (usually tens to hundreds of kHz), with the frequency or duty cycle of the switching being adjusted by a feedback loop (also widely referred to as a “compensation loop” or “feedback circuit”) to convert an input voltage to a desired output voltage. A SMPS may take the form of a rectifier (AC/DC converter), a DC/DC converter, a frequency changer (AC/AC) or an inverter (DC/AC).
The feedback loop typically comprises a controller that regulates the switching frequency or the switching duty cycle of the switching element(s) of the SMPS based on the output voltage or output current of the SMPS, in accordance with a control law defined by one or more control law parameters, to keep the output voltage of the SMPS in the vicinity of a predetermined value. For example, the SMPS may comprise a Proportional-Integral-Derivative (PID) controller that regulates the duty cycle (or the switching frequency, as the case may be) of the switching element(s) to keep the output voltage of the SMPS constant, in accordance with a PID control law that is characterised by the values of the P, I and D control parameters set in the PID controller.
When designing a digital compensation loop for a SMPS, use is often made of standard design tools that consider the design in the frequency domain, which can provide useful indications of system stability or robustness for plant changes and modelling errors. In this case, loop compensation is typically designed using rules of thumb for placing the PID compensation zeros, normally at the pole's natural frequency and one octave below. However, this approach usually requires the designer to have a good understanding of the system at hand, and in many cases yields sub-optimal solutions. Other approaches are based essentially on trial and error, which can be very time-consuming.